1. Field of the Invention
This invention relates to calorimetry, and more specifically to calorimetric measurements of working fluids flowing in a heat exchanger.
2. Background Information
Heat meters, often referred to as BTU meters or net energy flow meters, determine the heat energy flow of a heat exchanger by measuring the flow rate of the heat exchanger fluid and the difference in temperature across the heat exchanger and by multiplying those measured quantities by a factor selected to represent the specific heat of the fluid. The specific heat factor is generally selected based on a measured or presumed composition of the fluid and is corrected for changes in temperature.
In a practical application, such as an apartment building where a heat metered hydronic heating system has been installed, the composition of the water-based working fluid may not remain constant. This may be because of loss of some of the working fluid and replacement of it by a different fluid; because of chemical changes of one or more of the working fluid's components; or because of accumulation of debris in the plumbing system. As a result of any of these or other causes, the specific heat of the fluid can change from the value originally assumed or measured. When this occurs, the heat meter no longer measures heat transfer accurately.
Changes in the specific heat generally occur much more slowly than changes in flow rate. An exception to this occurs when a system is turned off, drained and refilled, in which case both the flow rate and specific heat may register a sudden change when the system is restarted.
Measuring the specific heat of a working fluid in heat exchanger is complicated not only by variations in temperature and composition, but also by the ambient fluid flow rate. A thermal transfer sensor will transfer more heat to a flowing fluid than to a stationary one, so any attempt to measure specific heat in a flowing system must provide either a measurement at zero flow rate, or an extrapolated value equivalent thereto. For example, Yamaguichi et al., in U.S. Pat. No. 5,983,700, teaches suddenly shutting off flow, making the specific heat measurement under actual zero flow conditions, and then restoring normal flow. This approach is not acceptable in systems that require continuous, uninterrupted heat transfer. Another approach, taught by Bonne et al. in U.S. Pat. No. 5,237,523, is to extrapolate a set of measurements to zero flow. These methods may result in a residual inaccuracy due to incorrect extrapolations.